Channel pressure, commonly referred to as aftertouch, is a powerful tool. Implemented intelligently, it lets you keep both hands on the keyboard while adding nuance to your playing. This month, we’ll look at ways aftertouch can be used for timbral control: On most instruments, it is easy to configure once you understand the possibilities.
Note that channel pressure affects all held notes equally on that MIDI channel. Polyphonic aftertouch, on the other hand, gives you independent modulation control over each note, though until recently, few instruments offered it. But that is changing as the new MIDI Polyphonic Expression (MPE) protocol increases in popularity, thanks in large part to the innovative and cost-effective products made by Roli, Roger Linn Design, and Haken Audio, among others. The following concepts can be used with both kinds of aftertouch.
THE BASIC SWITCHEROO
Typically in factory patches, aftertouch is assigned to the frequency cutoff of a filter, whereas the mod wheel is programmed to control LFO-based modulation, such as vibrato. For some players, reversing those assignments will feel more intuitive, giving them the ability to add filter swells with the mod wheel while controlling vibrato dynamically by leaning into the keys.
The Dave Smith Instruments Prophet 6 and OB-6 both include a dedicated aftertouch section on the front panel. While the Prophet 6 has independent routing options for the pitch of each oscillator, LFO depth, amplitude/volume, and filter cutoff, the OB-6 goes one step further with continuous control over the state-variable filter mode. This last trick sounds fantastic on pads, incidentally.
For fans of Ultravox, you can emulate Billy Currie’s trademark hard-sync sweeps (originally done on the Arp Odyssey) by turning on sync for VCO 1, then assigning aftertouch to control its frequency and setting the amount to maximum (see Figure 1). Now, when you lean into a note, you’ll get a nasty hard-sync sweep. Next, assign the mod wheel to control vibrato depth and add a touch of flanger and delay in the effects section.
Assigning aftertouch to alter the intensity of an FM modulator is another way to add timbral complexity to live performances. Ableton’s Operator includes MIDI controller routing in its algorithm window. To get started with this technique, begin with the default, stacked algorithm and assign aftertouch to increase the level (volume) of the first modulator (see Figure 2). Keep in mind that the modulator must have a sustaining envelope; that is, an envelope that doesn’t decay to zero, because once the modulator envelope reaches silence, there’s no way to increase its level.
In a synthesizer such as Xfer Records Serum, aftertouch can be assigned to nearly any destination within its mod matrix. Using aftertouch to control the wet/dry mix of an effect is one way to add emphasis to chords and leads. (This works particularly well with reverb.)
For unusual timbre shifts, map aftertouch to wavetable position and/ or warp depth. This allows you to do radical wavetable morphing or alter pulse-width modulation in real time (see Figure 3).
Once you’ve experimented with aftertouch assigned to these controllers and parameters, explore other configurations until you find the ones that fit your needs.